Diabetes, notably type 1 and type 2 diabetes, together with obesity, which is believed to be a major causal factor in the development of type 2 diabetes, constitute a major and even growing worldwide health problem. Diseases or disorders that may develop as a consequence of diabetes include cardiovascular and peripheral vascular disease, micro- and macrovascular complications, stroke and possibly certain forms of cancer.
Diabetes is characterized by a defective physiological regulation of blood glucose levels. Among the underlying conditions that may lead to diabetes are reductions in or the loss of pancreatic β-cell mass and function, with attendant reduction in or loss of endogenous Insulin production, and/or Insulin resistance (reduced sensitivity to insulin), i.e. reduction in or loss of the ability of endogenous insulin to bring about adequate regulation of blood glucose levels.
A number of hormones that lower blood glucose levels are secreted by the gastrointestinal mucosa in response to the presence and absorption of nutrients in the gut. These include glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), gastrin, and secretin.
GLP-1 [see, e.g., Ørskov, Diabetologia 35: 701-711 (1992)] is produced by tissue processing of proglucagon, a 180 amino acid peptide [see, e.g., Drucker, Diabetes 47: 159-169 (1998)]. The overall sequence of proglucagon contains the 29 amino acid sequence of glucagon, the 36 or 37 amino acid sequence of GLP-1, and the 34 amino acid sequence of glucagon-like peptide-2 (GLP-2; an intestinotrophic peptide).
The so-called exendins, which constitute another group of peptides that lower blood glucose levels, have some sequence similarity (53%) to GLP-1(7-36) [see, e.g., Goke et al., J. Biol. Chem. 268: 19650-19655 (1993)]. The exendins are found in the saliva of Helodermatidae species (beaded lizards). Exendin-3 is present in the saliva of Heloderma horridum (Mexican beaded lizard), while exendin-4 is present in the saliva of Heloderma suspectum (Gila monster). The amino acid sequence of exendin-4, which differs from that of exendin-3 at positions two and three, is HGEGTFTSDLSKQMEEEAVRLFIEWLKNGGPSSGAPPPS-NH2.
Exendin-4 has been reported to be a potent GLP-1 receptor agonist on isolated rat insulinoma cells [Goke et al., loc. cit.]. WO 99/07404 discloses that exendin-4 administered systemically lowers blood glucose levels by 40% in diabetic db/db mice, and a long-lasting blood glucose lowering effect of once-daily intraperitoneal injection of exendin-4 in diabetic ob/ob mice has also been reported [Grieg et al., Diabetologia 42: 45-50 (1999)].
U.S. Pat. No. 5,424,286 and WO 98/05351 disclose that exendin-3, exendin-4 and exendin agonists may be used for the treatment of diabetes, for reducing gastric motility and delaying gastric emptying, and for prevention of hyperglycemia, and WO 98/30231 further discloses that they may be used for reducing food intake.
The peptide hormone gastrin is secreted from cells in the gastric mucosa and from G cells in the duodenum, and among the major physiological roles of the hormone in humans are stimulation of secretion of gastric acid (i.e. HCl) and aiding in gastric motility. There are indications that gastrin may play a role in islet neogenesis, i.e. stimulation of insulin-secreting β-cell growth in the pancreatic islets [see, e.g., Korc, M., J. Clin. Invest., 92: 1113-1114 (1993); Rooman et al. Diabetes 51: 686-690 (2002)], and thereby contribute to regulation of blood glucose.
Gastrin shares receptors with another gastrointestinal peptide hormone, cholecystokinin (CCK). The receptors CCK-A R and CCK-B R have different affinities for gastrin and CCK variants. CCK-A R (or CCK R1) acts primarily as a receptor for sulfated CCK, whereas CCK-B R (or CCK R2) binds both CCK and gastrin equally well. CCK-B R is considered to be the “gastrin receptor” due to the higher levels of gastrin compared to CCK in plasma [Foucaud et al. Reg. Peptides 145: 17-23 (2008)].
CCK-B R can initiate several intracellular pathways upon binding of ligand, which is considered to be the reason for the diverse physiological roles of CCK. A key pathway downstream of CCK-B R is the MAPK (mitogen activated protein kinases) or ERK (extra-cellular regulated kinases) pathway, which is also activated by several growth hormones. Since CCK-B R is expressed in the pancreas, gastrin is able to contribute to cell proliferation and islet regeneration in this tissue.
In humans, gastrin occurs primarily in three forms, viz. gastrin34, gastrin17 and gastrin14 (with reference to the total number of amino acids in the sequence in question). Gastrin6 has also been identified. The shorter forms are generated by cleavage of C-terminally amidated gastrin34; thus gastrin17 consists of the last 17 C-terminal residues of gastrin34 (corresponding to progastrin (55-71), gastrin14 consists of the last 14 C-terminal residues of gastrin34 (corresponding to progastrin (58-71), and gastrin6 consists of only the last 6 C-terminal residues of gastrin34 (corresponding to progastrin (66-71). In human gastrin17 the N-terminal amino acid residue is a pyroglutamic acid (PyroGlu) residue. The amidated C-terminal 6 amino acids are the key receptor-binding residues of gastrin.